KEYWORDS: Ultrasonography, Fetus, Biometrics, 3D metrology, Reconstruction algorithms, Teleradiology, 3D image processing, 3D displays, 3D acquisition, Image quality
We have built upon an existing freehand 3D ultrasound imaging technique to enable display-less scanning at a local site by novice users and remote reading by integrating an electromagnetic tracker with a 2D probe. Seventy-two volumes are generated using a reconstruction algorithm from data collected by three users in a single longitudinal sweep across a 23- week fetus phantom in four different configurations for six scan durations ranging from 5-s to 30-s. The acquisition is semi-blinded: the user knows the fetal orientation but scans without image display and guidance of a conventional scan. Three non-expert readers and one expert Radiologist extract the clinically relevant planes and measure four key biometric features from the 3D images. In this paper, we propose (1) a risk metric R to rate the quality of the scan as a function of probe motion and contact and (2) a measurability index M for the availability of the 2D planes within the volume and visibility of the biometric features. Our analysis shows that R is the lowest and M the highest for 15-s acquisitions corresponding to an average transducer sweep speed of 2.4-cm/s. The finding is consistent with a reported speed range of 3-4 cm/s recommended for a low cost teleradiology solution for 2D ultrasound. The errors in average biometric measurements compared to the 50th percentile values in the fetal biometry tables for corresponding gestational week are within -3.8 to 5.7%. R, M, accuracy and precision of measurements are useful indicators of performance of the 3D ultrasound system.
Freehand 3D-ultrasound imaging using a 2D-ultrasound probe with attached position and orientation(pose) sensor is a cost-effective 3D-imaging modality. The standard method of 3D-reconstruction involves stacking the 2D-image slices in their appropriate position and orientations in the region swept by the probe. A single 2D-image is obtained by sequentially exciting the multiple elements of an array transducer probe. Standard 3D-reconstruction assumes that all the scanlines of a single frame are acquired concurrently. This assumption limits the speed of the scan required with accurate volume reconstruction. For correcting imperfections in reconstructed volume due to fast probe-motion, this paper proposes a new scanline-based 3D-volume reconstruction. This method corrects the placement and orientation of each scanline, such that the 'fast' probe motion does not distort the reconstructed volume. The improved performance of the proposed reconstruction method compared to standard plane-based reconstruction is demonstrated with scans performed with a convex probe on imaging phantoms. An anechoic cylindrical inclusion in a cube-shaped phantom is visualized using both reconstruction methods and visually compared to a reference image. Reference image of inclusion is reconstructed from a slow scan. For quantitative analysis, edge detected inclusions from images of the reconstructed volumes are compared with that of the corresponding reference image. The Dice coefficient of the inclusion with the proposed scanline-based reconstruction with the reference image is on an average 83% higher than that of the standard method.
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